An electrical generator used in the field of electrical power generation includes a stator winding having a large number of conductor or stator bars that are pressed into slots in a base body, in particular, a laminated stator core or a rotor body. Such an electrical generator represents a very expensive and long-term investment. Its failure not only endangers the power equipment itself but may also result in very severe service reduction due to the down time associated with repair. To avoid such a condition, increasing use is being made of a diagnostic system for early identification of defects. The diagnostic system furthermore allows a higher utilization level, making the power equipment more financially viable.
Due to the very high voltages within the generator, diagnostic systems for generators generally use sensor technology that avoids electrically conducting wires that could cause arcing to ground if they are deployed on a structure that is at a high voltage. As an alternative to conveying signals on conducting wires, sensing signals within generators have been conveyed by optical conductors, such as by glass fibers. For example, one known diagnostic system provides a vibration sensor on the end windings of the generator. In this form of system, the vibration sensor may comprise an accelerometer including a mass-spring component with light transmitting a signal, corresponding to the movement of the mass, through glass fibers. This type of system may provide an acceleration output signal that can be converted to deflections and analyzed for amplitudes and phases.
Although the signals from a vibration sensor are proportional to the stress in the material of the stator bar, the proportionality factor may be difficult to accurately determine. For example, the proportionality factor may vary depending on the design of the end winding and may further vary from bar to bar, such as may result from varying installation conditions including variations in the tightness of each stator bar within a respective slot.
In a further approach to determining vibration associated with the stator bars, an optical fiber sensor including a Bragg grating may be located on ripple springs adjacent stator bar ends to detect changes in the frequency of signals received from the sensor. The detected changes may be indicative of a change of elasticity of the ripple spring and/or other conditions that may cause the stator bars to move. Such a monitor is disclosed in Patent Application Publication US 2008/0036336 A1.
It is preferable to obtain strain measurements directly on stator bars to provide a direct indication of the stress applied to a particular stator bar. In addition, for long term condition based maintenance it may be desirable to further obtain information from a strain measuring sensor indicative of strain on stator bars in combination with information from a vibration measuring sensor indicative of an amplitude of vibration of stator bars to provide data that may be used to track and/or predict fatigue of parts in the stator core.